CALCIUM AND MAGNESIUM CHLORIDE ATTACK ON CEMENT - BASED MATERIALS: FORMATION, STABILITY, AND EFFECTS OF OXYCHLORIDES

摘要

Calcium and magnesium oxychloride crystals of the form 3Ca(OH)_2.CaCl_2.12H_2O and 3Mg(OH)_2.MgCl_2.8H_20 respectively were obtained by the reaction between calcium hydroxide leached from mortar bars and either a 15% calcium chloride solution or a 14 to 20% magnesium chloride solution respectively. These crystals, which caused significant expansion and cracking of mortar, were found to be very unstable when subjected to changing environmental conditions including those normally encountered in typical laboratory sample preparation (e.g., drying and elevated temperatures). Under all conditions there was a tendency for the crystals to spontaneously lose some or all of their combined water resulting in changes to the crystal structure as identified by x-ray diffraction and thermogravimetric analyses. This instability may be one of the reasons why few studies have managed to correlate the deterioration of cement-based materials with the formation of oxychlorides.